1. Field of the Invention
This invention is directed to a subsurface safety valve for oil and gas wells. The invention is particularly useful in a high pressure, high temperature well environment which is becoming more common due to the increased depth of many wells being drilled today.
2. Description of Related Art
Current subsurface safely valves normally include an upper tubular member 29, a valve body housing 15, and a lower tubular member 10 as shown in FIG. 1. A flapper type valve 13 is located in the valve body housing. Threaded connections are located at the juncture of the upper and lower tubular housing for connection to the valve body housing. Upper tubular member 20 includes a bore 9 in which a piston 22 is located for axial movement. A change in the pressure above and below the piston causes the piston to move, which in turn allows a compressed spring 18 to force a valve tube 24 upwards. This movement allows the flapper valve to deploy inwardly over a valve seat 52 in the valve body to prevent flow of fluid to the surface. Such an arrangement is shown in FIG. 1.
As wells are drilled deeper and deeper, the temperatures and pressures increase. Therefore it is necessary to design subsurface safety valves that will not fail under high temperatures and pressure conditions.
One solution with respect to the treaded couplings is simply to make thicker tubular housings. However, this solution has two serious adverse drawbacks. For safety valves operating at pressures around 10,000-15,000 psi the original load bearing shoulder geometry was determined by the outside diameter of the box connection. The shoulder was sufficiently loaded to provide high break-out torque levels, that is the contact pressure was intentionally designed to be high so the threads would have a break-out torque higher than the make-up torque. Also the normal connection allowed more radial compliance for an adequate metal-to-metal seal. Compliance is important to insure that the seal can be maintained under varying temperature and load conditions. Thicker tubular connections will result in lower contact pressure on the load bearing surface due to the increased contact area at the same makeup torque. This will decrease the break-out torque. Secondly, the compliance of the metal to metal seal will be significantly decreased which will lead to external pressure leakage problems.
Another area of concern with increasing pressure and temperature is the piston bore. The portions of the upper tubular member near the inner and outer regions around the piston bore are subject to increased stresses which cause failure at these areas.